The frequency and power intensity spectra of the light emitted by semiconductor lasers employed in optical systems can be altered when reflected light impinges upon the lasers. Such alterations are undesirable because they can lead to errors in the detected information. Thus, efforts have been made to develop devices, called optical isolators, for isolating the semiconductor lasers from reflected light. An optical isolator based on rotation of linearly polarized light is exemplified by a bulk magnetic garnet material, e.g., bulk single crystal yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12, called YIG), positioned between a polarizer and an anlyzer. In operation, a magnet is employed to magnetize the YIG (in the direction of light propagation). Light emitted by a laser and linearly polarized after transmission through the polarizer is directed into the YIG material. Under the influence of the net magnetic moment within the (magnetized) material, the linearly polarized light experiences circular birefringence. As a consequence, the light remains linearly polarized, but the polarization direction is continuously rotated in either the clockwise or counterclockwise direction as the light traverses the material. If the material is of appropriate dimension, the polarization direction is rotated through, for example, 45.degree. and the light is thus transmitted by an analyzer set at 45.degree.. Reflected light transmitted by the analyzer also enters the YIG material and also undergoes a rotation of 45.degree. in the same direction as the light which originally traversed the material. Consequently, reflected light, after traversing the YIG material, is polarized at 90.degree. to the polarizer, and is thus precluded from impinging upon the laser. (The phenomenon by which a fixed length of magnetized material rotates both forward and backward propagating linearly polarized light by the same amount and in the same direction is denoted antireciprocal magneto-optical rotation. Devices which include such materials are referred to as antireciprocal devices. By contrast, an optical element that rotates oppositely propagating beams of light in the same direction, but not necessarily by the same amount, is said to be "non-reciprocal.")
Although antireciprocal light-rotating devices based on bulk materials are useful, thin-film-waveguide light-rotating devices are advantageous where incorporation in miniaturized integrated optical devices is envisaged. For example, a thin film optical isolator using planar magnetization would readily permit the use of guided wave optics (and thus eliminate the need for focusing lenses) and could also serve as a building block for integrated optical devices.
Thin film waveguiding devices employing planar magnetization have, in fact, been fabricated. Such devices have included, for example, a magnetized (in the plane of the film) layer of YIG epitaxially grown on a (closely lattice matched) substrate of, for example, gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12, called GGG). While these devices are potentially attractive, they are, unfortunately, subject to linear birefringence. Linear birefringence means that the TE and TM components of the light see different refractive indices, resulting in one of these components propagating through the film at a faster speed than the other. Thus, when traversing a magnetized thin film, e.g., a magnetized layer of YIG, light is subjected to a birefringence which includes both a linear component and a circular component. As a consequence, reflected light is incompletely blocked. Thus, the effects of linear birefringence in thin film, magnetized, waveguiding devices have presented a serious obstacle to their advantageous use.
The factors responsible for the linear birefringence found in thin films of, for example, YIG have been identified. One of these factors is what is here termed shape linear birefringence, which is due to the presence of discontinuities in refractive index at the film-air and film-substrate interfaces. A second factor responsible for linear birefringence, commonly termed stress-induced linear birefringence, is due to a lattice mismatch between the film and the substrate. This mismatch subjects the film to either a compressive or tensile stress in the plane of the film, which, like shape linear birefringence, has the effect of inducing a refractive index anisotropy in the film. A third factor responsible for linear birefringence, commonly termed growth-induced linear birefringence, is due to a non-random distribution of certain ions in the film crystal lattice, produced by the conventional techniques used to epitaxially grow films on substrates. In many cases, the sign of the stress-induced and/or growth-induced linear birefringence is opposite to that of the shape linear birefringence. Thus, these different sources of linear birefringence can be used to cancel each other to produce zero net linear birefringence.
For example, R. Wolfe et al, "Thin-Film Garnet Materials with Zero Linear Birefringence for Magneto-Optic Waveguide Devices (Invited)", J. Applied Phys., Vol. 63, pp. 3099-3103, (1988) describes a method for fabricating a thin film, waveguiding, polarization rotator which achieves essentially zero net linear birefringence, i.e., achieves a value of the dimensionless ratio B/F less than or equal to about 0.1. The ratio B/F expresses the ratio of linear birefringence to Faraday rotation. B is equal to .DELTA..beta./2, where .DELTA..beta.=2.pi..DELTA.n/.lambda. and .DELTA.n denotes the difference in the refractive indices seen by the TE and TM components, while .lambda. denotes the wavelength of the light. Physically, .DELTA..beta. is the phase difference (induced by the net linear birefringence) between the TE and TM components per unit length of film, and has dimensions of, for example, radians per centimeter. In addition, F denotes the Faraday rotation per unit length of the film. F is expressed in the same units as .DELTA..beta., e.g., in radians per centimeter.
Wolfe et al reported that in epitaxial garnet films, .DELTA.n can be reduced to small values by (1) growing single-mode multilayer films to minimize the shape effect, (2) growing the films in compression to control the stress-induced effect, and (3) annealing at high temperatures to eliminate the growth-induced effect. The remaining birefringence can be reduced to zero by growing the top active layer so thick that the shape effect is smaller in magnitude than the stress effect, and then thinning it by chemical etching until the effects exactly cancel each other at a particular wavelength and temperature. An alternative method is to begin with a thin top layer such that the magnitude of the shape effect is relatively large, and then to deposit a dielectric layer such as silicon nitride of the proper thickness to reduce the shape effect so that it exactly cancels the stress effect.
By the method of Wolfe et al, linear birefringence at a given temperature and at a given wavelength can be essentially eliminated from a thin-film magnetic waveguide. If the waveguide is, moreover, composed of a non-reciprocal material, a useful non-reciprocal optical device such as an isolator is readily produced.
However, it may be necessary, in practice, to operate the waveguide over a range of wavelengths and temperatures, and hence it may be necessary to tolerate a small but significant amount of linear birefringence in the waveguide.
H. Dammann, et al, Abstract: "The 45.degree. Waveguide-Isolator," Journal of IOOC, July 1989, Kobe, Japan (to be published) have described a method of using a thin-film optical isolator that achieves useful optical isolation in the presence of linear, as well as magnetic circular, birefringence. Dammann, et al, observed that despite the presence of linear birefringence, light that enters such a waveguide in a linear polarization state will always exit in a linear polarization state, provided that at the midpoint of the waveguide, the major axis of the polarization ellipse is parallel or perpendicular to the major surface of the waveguide. (This condition is here referred to as the Dammann condition.) Significantly, magnetic materials having linear birefringence are not, in general, anti-reciprocal, although they may be non-reciprocal. As a consequence, although a non-reciprocal waveguide can readily be provided that satisfies the Dammann condition for light propagating in one direction, the forward beam and the reflected (reverse-propagating) beam will not, in general, simultaneously satisfy the condition.
Thus, for example, an optical isolator using Dammann's principle is advantageously made by providing a non-reciprocal, 45.degree. optically rotating waveguide. For illustrative purposes, it is assumed that linearly polarized light enters such a waveguide through an input polarizer oriented at 67.5.degree. from the TE mode orientation (considered to correspond to 0.degree.) and exits through an output polarizer oriented at 22.5.degree.. In a practical isolator, it is typically important to minimize the amount of reflected light that escapes in the reverse direction, even at the expense of suffering some loss in the forward direction. In order to assure that the reflected light is maximally blocked by the input polarizer (oriented at 67.5.degree. to the TE axis), the waveguide is designed such that the reflected light, rather than the forward-propagating light, satisfies the Dammann condition.
That is, in general, the forward-propagating light arrives at the output polarizer in an elliptical polarization state. A portion of this light is resolved and transmitted by the output polarizer. Reverse-propagating light (i.e., light reflected by any discontinuity in the optical path) passes through the output polarizer and starts out with linear polarization at 22.5.degree.. This light satisfies the Dammann condition. That is, at the midpoint of the waveguide, the polarization ellipse has a major axis at 0.degree., and the light arrives at the entrance polarizer linearly polarized at -22.5.degree.. This light is completely blocked by the input polarizer, giving essentially perfect reverse extinction.
Provided the amount of linear birefringence in the waveguide is relatively small, the forward-propagating light that is incident on the output polarizer will have a relatively large component transmissible by the output polarizer, and the waveguide will be useful as a practical optical isolator. However, the greater the linear birefringence, the greater the loss at the output polarizer is likely to be.
Because linear birefringence is known to be sensitive to wavelength and temperature, it has until now been believed that the method of Dammann, et al, is useful only for operation at essentially a single optical wavelength, and within a narrow, carefully controlled temperature range.
The effect of temperature on magnetic thin film optical isolators has been discussed, for example, by J. P. Castera et al, "Phase Matching in Magneto-Optic YIG Films by Waveguide Temperature Control," Electronics Lett., Vol. 25, p. 297 (1989). Castera reported the construction of a waveguide isolator that could be tuned to essentially zero linear birefringence by using temperature to alter the stress-induced component of the linear birefringence. Significantly, Castera reported that because of the sensitivity of the birefringence to temperature, such a tuned device would require temperature stabilization. For example, the temperature would have to be maintained within a 2.degree. C. range in order to achieve a stable isolation of 30 dB.
The wavelength sensitivity of such isolators has been discussed, for example, by R. Wolfe, et al, "Etch-Tuned Ridged Waveguide Magneto-Optic Isolator," Appl. Phys. Lett., Vol. 56, p. 427 (1990). Wolfe reported that when pure TE light was injected into an etch-tuned waveguide isolator, the isolation ratio changed from a desirable value of -35 dB at the tuning wavelength of 1.545 .mu.m to a much less desirable value of -16 dB at 1.45 .mu.m. Thus, because of the wavelength sensitivity of the birefringence, the isolation ratio fell in magnitude by 19 dB over a wavelength range of less than 0.1 .mu.m.